The worst sledding accident I had was in college when we sledded down the side of a highway off-ramp, pretty close to a 30-degree angle, on fiberglass cafeteria trays. The snow was actually solid ice you couldn't even punch through with your boot. A small tree got in my way, I bailed left and my sled went right. I hit that tree so hard right in the gut that my knees hit my forehead. I know I busted a rib or two because I couldn't sit up from any lying position for about 2 weeks. I thought I ruptured my spleen.
My brother in law from CA went sledding last winter by us, hit the first ice ramp, lost the sled, then landed HARD on his tailbone ONE THE SECOND RAMP. He was just laying there for about 5 minutes. I had to literally drag him off the run before he got run over.